Upon start up I have an rpm related knock between 2000 and 2500. It doesn't sound like a valvetrain issue. It pretty much goes away when its warmed up. I'm also experiencing lower rpm bucking and stalling under load which can be diagnosed as knock sensor picking up the knock from the out of tolerance part.

Note, its fair to say the problem is not bad and the stalling is intermittent.

Something to be worried about? If it is indeed main bearings going out, how long will they last? The noise did not worsen after a severe overrev and its done it the same since I bought the car.